1. Field of the Invention
The present invention generally relates to a power transmission and, more particularly, to a manual power transmission for varying the gear ratio between the automotive engine and drive wheels.
2. Description of the Prior Art
A manual power transmission though orthodox it may be nowadays has again gained popularity because of fun of car maneuverability. Of various manual power transmissions available, the power transmission is well known which comprises an input shaft drivingly coupled with the automotive engine, an output shaft drivingly coupled with the drive wheels and coaxially aligned with the input shaft, a countershaft or layshaft disposed parallel to those shafts, a plurality of speed gear pairs including gears on the input and output shaft and associated countergears on the layshaft, and a reverse idler shaft disposed parallel to any of those shafts and supporting thereon an intermediate idler gear engageable with a reverse-drive gear and a reverse-drive countergear mounted respectively on the input shaft and the layshaft for transmitting the rotation of the input shaft to the layshaft.
The power transmission of the structure described above is generally of a construction in which the drive of the input shaft is transmitted to the layshaft with the rotational speed (rpm) of the input shaft having been adjusted at a predetermined gear ratio determined by mutually meshed speed gears on the input shaft and the layshaft, respectively, and is in turn transmitted from the layshaft to the output shaft at another predetermined gear ratio which varies with the selected gearshift position. This transmission is known as an input reduction gear type that is generally characterized in that reduction in rotational speed takes place between the input shaft and the layshaft.
In this known input reduction gear type, the speed gears are necessarily mounted fixedly on the input shaft and the layshaft, respectively. The rotational speed of the layshaft is reduced according to the gear ratio of the speed gear pair intervening between the input shaft and the layshaft and such gear ratio is substantially fixed regardless of the gearshift position. In other words, the rotational speed of the layshaft is lower than the rotational speed (rpm) of the input shaft regardless of the gearshift position.
On the other hand, a different type of the power transmission is also known in which reduction in rotational speed takes place between the layshaft and the output shaft. This type is known as an output reduction gear type, an example of which is disclosed in, for example, the Japanese Laid-open Patent Publication No. 2-93151, published Apr. 3, 1990 which corresponds to U.S. Pat. No. 5,014,567, issued May 14, 1991. In this known output reduction gear type, since the torque of the input shaft is transmitted to the layshaft without being increased, an input load acting on the various speed gears can be minimized. Also, since no fixed speed gear pair intervene between the input shaft and the layshaft, maneuverability of the gear shifting mechanism will not be affected by any selected gear ratio represented by the speed gear pairs and, therefore, the force required to accomplish a gear shifting can be advantageously minimized. In addition, obnoxious sounds generated by clashing of the gears during the engine idling condition can advantageously be reduced substantially.
In this conventional output reduction gear type, the rotational speed of the layshaft is subject to variation according to the gear ratio of the gear pair selected according to the gearshift position and will, at a certain high-speed gearshift position or positions, attain a value higher than the rotational speed of the input shaft. Thus, for a given gear ratio, the rotational speed of the layshaft is considerably higher at a certain high-speed gearshift or positions in the output reduction gear type than that in the input reduction gear type.
As discussed above, the output reduction gear type has the layshaft that tends to be driven at a higher speed than the input shaft. This feature appears to have brought about the following problems. Particularly, since the rotational speed of the layshaft is extremely high at higher speed gearshift positions as discussed above, speed gears of the various gear pairs which are rotatable idle relative to the shaft on which they are mounted for rotation independent thereof have their associated bearings placed in a difficult lubricating condition during their idle rotation. In particular, the idler gear forming a part of the reverse-drive gear pair, that is, the reverse-drive idler gear, is driven by an intermediate idler gear in a direction counter to the direction of rotation of the shaft supporting the reverse-drive idler gear and the difference in rotational speed relative to such shaft tends to increase considerably. In addition, so far as the first-speed gear pair and the reverse-drive gear pair each having a relatively high gear ratio are concerned, and in particular where one of each gear pair having a smaller gear diameter than the other is utilized as an idler gear, the respective idler gears of those gear pairs will be driven idle relative to the input shaft at a considerably high velocity at the high-speed gearshift position or positions, and therefore, the bearings associated therewith tend to be similarly placed in a difficult lubricating condition, or otherwise the associated bearing for those idler gears will be undesirably seized.
As is well known to those skilled in the art, in the gear-type power transmission regardless of the type, supply of lubricant oil to respective meshing regions of the gears of the various gear pairs, the associated bearings for the idler speed gears and/or any other portions that require oiling is generally carried out by stirring the lubricant oil, accommodated in an oil reservoir at the bottom of the transmission housing, upwardly by means of some of the countergears on the layshaft then driven together with the latter. Most of the lubricant oil so stirred upwardly collides against the interior wall of the transmission housing and subsequently falls by gravity along the interior wall of the transmission housing and is cooled in contact with the interior wall of the transmission housing before it is again supplied to the meshing regions, bearings and/or portions that require oiling.
Where the lubricant oil is to be supplied in the manner described above, the temperature of the lubricant oil has to be taken into consideration as it is affected by the stirring resistance of the lubricant oil imposed by the rotation of both of thelayshaft and the countergears. In order to reduce the stirring resistance to thereby suppress an increase of the lubricant temperature, it is desirable to reduce the amount of the lubricant oil used to a value as small as possible, not exceeding the required amount. Since in the case of the output reduction gear type, the rotational speed of the layshaft is considerably higher at a certain high-speed gearshift position or positions than that in the input reduction gear type as hereinbefore discussed, it is strongly desired that the amount of the lubricant oil be reduced.
However, the need to reduce the amount of the lubricant oil and the assured supply of the lubricant oil to required locations pose an incompatible problem which has hitherto been considered difficult to solve.